Interpretive Summary: Hydramethylnon is an important insecticide for controlling German cockroaches. Anecdotal evidence has indicated that German cockroaches may be developing insecticide resistance toward hydramethylnon. Unfortunately, traditional residual or topical bioassays are not effective methods of evaluating hydramethylnon resistance development because it is a stomach poison. As part of a Cooperative Agreement with the American Cyanamid Company, scientists at the Center for Medical, Agricultural and Veterinary Entomology, Gainesville, Florida have developed a novel bioassay method capable of identifying physiological and behavioral hydramethylnon resistance in German cockroaches. Results indicate that the majority of German cockroach strains collected recently from the field that possess resistance to most traditional insecticides were not physiologically resistant to hydramethylnon. However, several strains exhibited behavioral resistance.

Technical Abstract:
A novel consumption bioassay was employed to evaluate hydramethylnon tolerance among 14 insecticide-resistant and 1 insecticide-susceptible strain of German cockroach, Blattella germanica (L.). The feeding bioassay provided the ability to assess physiological changes in hydramethylnon tolerance while simultaneously evaluating German cockroach avoidance behavior toward Siege gel bait. The cockroach strains, which had been shown previously to be resistant to pyrethroid, carbamate, and organophosphate insecticides, were bioassayed in two groups. The first group was bioassayed by the dose response method and the second group was assayed with a single diagnostic dose. All of the strains evaluated by the dose response consumption bioassay method were more susceptible to hydramethylnon than the Orlando laboratory strain. Although the dose response bioassay data did not indicate physiological resistance among these strains, behavioral resistance (avoidance) was implicated in the Union 511 and Malo strains. In a second group of insecticide-resistant strains, physiological, as well as behavioral resistance to hydramethylnon was implicated when cockroaches were challenged with a diagnostic dose of hydramethylnon in Siege . At the Orlando LD99 (3.5 æg hydramethylnon per cockroach) only 80 and 75% of the NASJAX and Hyd-Sel adult males consumed the bait, respectively. These data indicate that avoidance behavior may be developing in these strains. Furthermore, physiological resistance may be present in the Hyd-Sel strain, as only 89% were killed when fed the LD99 Orlando dose.